This invention relates to electrostatic duplicating or copying processes and apparati useful therein. More particularly, the invention relates to the ambient temperature fixing of pressure fixable imaging powders to a receptor such as a sheet of paper, and provides a method and an apparatus for permanently fixing such imaging powder to such receptor to produce photocopies that have substantially reduced image and background gloss.
Electrostatic copying processes generally comprise the steps of electrostatically charging a photosensitive substrate, exposing the charged substrate in an image-wise manner, developing the charged image areas with imaging or toner powder, and then permanently fixing the toner powder. In the so-called "transfer process" a permanent photosensitive substrate is employed and the developed image areas are transferred to an expendable receptor, e.g. paper, before the toner is permanently fixed. In the so called "direct process", the substrate and the receptor are the same and comprise an expendable electrostatic copy paper and the toner is permanently affixed to the copy paper without transfer. The present invention provides a method and apparatus for fixing toner powders in both these processes.
Pressure-fixable toners become permanently affixed to the receptor by the application of pressure. These powders generally require that a minimum fixing pressure be applied thereto before the imaging powders are permanently fixed to the receptor. An apparatus which fixes imaging powders by application of pressure must be capable of exerting a pressure on the powder particles in excess of the minimum fixing pressure to cause the particles to coalesce and to fix (i.e., to bond) to the receptor. Generally this pressure is applied by a pair of steel fusing rolls. Most commonly these rolls have a fine surface finish. Pressures in the range of 25 to 400 pounds per lineal inch (4.5 to 71 kg/lineal centimeter) are often required to pressure fix imaging powders at ambient temperature. Such pressures are difficult to maintain and produce fixed images and substrates that are shiny or glossy, thus making the copy difficult to read and aesthetically unpleasant.
Many attempts have been made to eliminate this problem. Usually the attempts take the form of reducing the amount of pressure applied to the fixing rolls. This, however, has not proven entirely satisfactory. Thus, while the non-image or background areas exhibit some reduction in gloss, the image areas exhibit virtually no reduction in gloss. Additionally, the image areas are less permanently fixed to the receptor surface.